The present invention releates to silicone paper release coating emulsion compositions and more particularly, the present invention relates to paper release silicone coating emulsion compositions which can be cured either with a tin catalyst or a platinum catalyst.
Silicone paper coating compositions are well known. Traditionally, such systems comprise a silanol terminated diorganopolysiloxane polymer, a hydride polysiloxane cross-linking agent and a tin salt of a carboxylic acid as the catalyst. Other additives were added to these ingredients and the total ingredients were then emulsified with the appropriate emulsifying agents in water to prepare a silicone paper treating emulsion. The resulting emulsion was then applied to paper using various types of equipment so as to apply a coating of silicone of about 1 to 3 mills thick on the paper.
As so treated, such paper had very good release properties to pressure sensitive adhesives. Accordingly, such paper could be applied as release backing over pressure sensitive adhesive tapes and when desired the paper could be easily stripped off from the pressure sensitive adhesive tape. Such pressure sensitive adhesive tapes and the paper release tapes appended to the adhesive tapes has widespread used in the home and industry.
Accordingly, the need for silicones and other materials to make paper releasable to pressure sensitive adhesive tapes is in great demand. Silicones are especially sought after as paper release coatings because they are non-toxic, they are inert to the pressure sensitive adhesives and also provide generally, a good release surface for pressure sensitive adhesives. Accordingly, the demand for silicones for this application has increased from year to year. In line with such increase in demand of silicones for such application there has developed a continued effort into obtaining better and better silicone paper release coatings.
There was several criterion that were developed for the evaluation of silicone paper coating compositions so as to produce good paper release coatings. Such criterion was that the silicone coating should have the proper flow properties so it could be applied with existing equipment and that the silicone coating should be such that the emulsion did not wet out the paper.
Another desirable property for the silicone coating was that it should have some abrasion resistance, that is it would not rub off when it was rubbed so that it would be maintained on the surface of a paper to act as a paper release coating. Also the coating had to cure properly in the time allotted for the cure of the coating within industrial limitations and did not require a post-bake cure, thus decreasing the time necessary to process the release coated paper. The tin curing composition as mentioned previously, was able to meet many of the above criteria. However, while inexpensive, it did require a post cure and in addition, such tin catalyzed silicone coatings also did not have as good abrasion resistances as would be desired.
Accordingly, one improvement in this area was the development of SiH olefin platinum catalyzed silicone compositions as coating compositions for paper. Such SiH olefin compositions generally comprised a vinyl-terminated diorganopolysiloxane polymer, a hydride polysiloxane cross-linking agent and a platinum complex catalyst. While such platinum cured paper release coatings were more expensive than the tin catalyzed versions, they had the advantages that they were faster curing and required no post-bake cure. However, it was found that in some cases that such platinum catalyzed paper release coatings did not have as good abrasion resistance as would be desired. Such platinum catalyzed coatings were formulated in the form of emulsions, since emulsions were easier to apply by traditional equipment.
Presently, there has been developed solventless systems both of the tin catalyzed version and the platinum catalyzed version. A disclosure of such solventless systems is to be found in the patent application of Maurice E. Grenoble Ser. No. 748,738 filed Dec. 9, 1976 presently pending before the Patent Office. The advantage of such solventless systems is that they eliminate the need of emulsifying agents and solvents to produce the desired emulsions, while solvents and emulsifying agents would evaporate into the atmosphere when the paper release coating was dried, thus possibly creating pollution problems. Accordingly, for the extingency of where the evaporation of the solvent and emulsifying agent vapors might cause pollution problems in a paper treating plant, the solventless systems were developed and have been found satisfactory in this respect. However, such solventless systems have not gained widespread use in the industry as yet, since they require special equipment to apply versus the emulsion systems.
Accordingly, as stated previously, it may be desirable to utilize in a paper treating plant two systems for producing paper release coatings where it is desired to have an inexpensive system in which the optimum of adhesive release properties are not that important and one in which a post-bake cure can be utilized. On the other hand, in the same manufacturing plant it may be desired to treat the paper with the paper release coating which has the optimum of release properties, which requires no post-bake cure, and which has good abrasion resistance. Accordingly, as can be envisioned in the same manufacturing plant, it is desirable at that time to switch from one paper release coating system to another. In such plants it was found that after the tin system had been used, that extra care had to be taken to completely clean the tin out of the equipment, otherwise, the tin would poison the platinum if the platinum system was used subsequently. In addition, it was found that in going from one system, the entire ingredients there were to be used to treat the paper would have to be changed in going from a tin catalyzed paper release coating system to platinum catalyzed silicone paper release system. Accordingly, it was highly desirable to provide or develop a single system which could be cured with either a tin catalyst or a platinum catalyst in which the system would not be become poisoned by the tin if it was decided to subsequently use platinum in the system.
It was felt that such system could be devised by having the base polymer, a diorganopolysiloxane polymer which was silanol terminated and which had vinyl in the polymer chain. A system disclosing such a base polymer is that to be found in Moeller U.S. Pat. No. 4,008,346, which is incorporated by reference into the present case. This patent discloses a paper release coating emulsion comprising a vinyl containing base polymer having silanol terminal groups, a hydride cross-linking agent and a platinum catalyst. The distinction in this invention of the prior art is that the base polymer is produced by emulsion polymerization so that it is not necessary to emulsify the base polymer to produce the paper coating emulsion. It was determined by such emulsion polymerization the viscosity of the base vinyl-containing polymer would have to have a viscosity in the range of 25,000 centipoise to 1 million centipoise at 25.degree. C. When it was attempted to cure such a polymer, that is the silanol terminated diorganopolysiloxane polymer of the Moeller Patent having vinyl in the polymer chain, it was found that such a polymer would not cure with a tin catalyst. It would only cure with a platinum catalyst. As pointed out previously, the reason for the high viscosity of the vinyl-containing base polymer is the emulsion polymerization by which it is produced as disclosed in the foregoing Moeller Patent. Indeed, it was felt it was because of this high viscosity of the polymer that such a polymer did not have a sufficient silanol content such that it could be cured with a tin catalyst.
Accordingly, it was unexpected that a silanol terminated diorganopolysiloxane polymer having vinyl on the polymer chain, which was not produced by emulsion polymerization, but which was emulsified could be cured with either a tin catalyst or a platinum catalyst to produce an acceptable paper release coating composition.
It should be noted that while the Moeller coating was an advantageous type of coating for paper release coating applications, because it was emulsified in process and it could still not be cured with a tin catalyst and further did not have a very good abrasion resistance, that is in many cases, unless the composition was post-cured the coating would smudge or rub off when it was rubbed with a finger.
Accordingly, it is one object of the present invention to provide for a silicone paper release coating composition which can be cured either with a tin catalyst or can be cured with a platinum catalyst.
It is an additional object of the present invention to provide for a silicone paper release coating composition which, when cured with a tin catalyst or cured with a platinum catalyst has good abrasion resistance.
It is an additional object of the present invention to provide for a process for producing a silicone paper release coating composition which can be cured with either a tin catalyst or a platinum catalyst.
It is yet an additional object of the present invention to provide for a process for coating paper with a paper release coating composition which can be cured with either a tin catalyst of a platinum catalyst. These and other objects of the invention are accomplished by means of the disclosure set forth hereinbelow.